It's a market depth chart - the orange line on the left represents the cumulative standing buy orders, the blue line represents the standing sell orders, the gr

It's a market depth chart - the orange line on the left represents the cumulative standing buy orders, the blue line represents the standing sell orders, the green line is the historical price per trade with the latest trade (thus value) being at the bottom. The Y axis is a representation of the bitcoin value, and the X axis is the representation of the USD buy/sell prices.